Stents are cylindrical-shaped devices that are radially expandable to hold open a segment of a vessel or other anatomical lumen after implantation into the body lumen. Various types of stents are in use, including expandable and self-expanding stents. Expandable stents generally are conveyed to the area to be treated on balloon catheters or other expandable devices. For insertion, the stent is positioned in a compressed configuration along the delivery device, for example crimped onto a balloon that is folded or otherwise wrapped about a guide wire that is part of the delivery device. After the stent is positioned across the lesion, it is expanded by the delivery device, causing the length of the stent to contract and the diameter to expand. For a self-expanding stent, commonly a sheath is retracted, allowing expansion of the stent.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications, including intravascular angioplasty. For example, a balloon catheter device is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion, thereby increasing the inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow.
Soon after the procedure, however, a significant proportion of treated vessels restenose. To prevent restenosis, short flexible cylinders, or stents, constructed of metal or various polymers, are implanted within the vessel to maintain lumen size. The stents act as a scaffold to support the lumen in an open position. Various configurations of stents include a cylindrical tube defined by a mesh, interconnected stents or like segments. Some exemplary stents are disclosed in U.S. Pat. No. 5,292,331 to Boneau, U.S. Pat. No. 6,090,127 to Globerman, U.S. Pat. No. 5,133,732 to Wiktor, U.S. Pat. No. 4,739,762 to Palmaz and U.S. Pat. No. 5,421,955 to Lau.
Stent insertion may cause undesirable reactions such as inflammation, infection, thrombosis, and proliferation of cell growth that occludes the passageway. Stents have been used with coatings to deliver drugs or other therapeutic agents at the site of the stent that may assist in preventing these conditions. The coatings must be bioengineered to control the release of highly potent and potentially toxic drugs.
The choice of a coating may be limited by its mechanical attributes. For example, a polymer with a desired drug elution profile may be brittle and prone to cracking in areas of the stent that experience high strain while the stent is being compressed during manufacture or expanded during delivery into the target vessel. Alternatively, a coating that erodes at a desired rate, thereby delivering drug at a known, controlled rate, may also be brittle or otherwise unable to withstand the strains of compression or expansion. Cracking of a coating in high-strain areas may cause extensive delamination of the coating from other areas of the stent, resulting in an unknown amount of drug being delivered.
Therefore it would be desirable to have an improved, differentially coated stent that accommodates these areas of high strain and overcomes the above and other disadvantages.